Heretofore, for the driving mechanism of a needle bar unit and work feed unit for stitch pattern forming in an electronic sewing machine, either a linear actuator controlled by a servo-amplifier or stepping motor (pulse motor) controlled by a drive circuit have been utilized. In said linear actuator, the center leg is inserted in said actuator in parallel with permanent magnets which are fixed to opposite sides of the casing facing each other, so that the magnetic path of said permanent magnets passes through said casing and said actuator center leg. A bobbin coil is mounted on the center leg so loosely as to be slidable on said leg back and forth. A protruding lug is integrally molded with the above coil and it is coupled to the rotor axis of a potentiometer contact through the medium of a lever which is pivotally adjustable on both the lug and center axis of the potentiometer by a screw. When an analog DC potential representing a command signal corresponding the desired stitch pattern information is applied to this coil through the output of said servo-amplifier, the said bobbin coil is driven forward or backward in proportion to the magnitude of the stitch pattern signal potential and polarity. This movement is transmitted to the potentiometer through said lever which is fixed on the center axis of potentiometer. The center axis of said potentiometer is rotatably driven by said lever. As a predetermined potential is applied to the two terminals of said potentiometer from a stabilized DC source, the angle displacement of the center axis can be detected by the potential of the center contact. This potential is then fed back to the servo-amplifier for summation with the inverse phase of the input command signal potential so that the position servo functions of the needle bar and work feed unit are maintained. In addition, it can be corrected by an added potential derived from differentiating the detected signal.
However, on this linear actuator, if the electromotive force derived from the current through the coils, which are inserted in a magnetic field generated by opposing permanent magnets, is required to be increased, it is necessary to either increase the number of coil turns on the bobbin, increase the driving current or provide a larger volume of permanent magnet or magnetic material with a higher coercive force, but all of which are expensive and make this device higher in cost. Also, when a larger current flows, it causes more heat dissipation in the unit which raises the internal temperature to an undesirably high value. Further, the potentiometer is inconveniently located for outside adjustment of the unit.
On the other hand, when a stepping motor is used, it is shifted by a digital step angle related to the stitch pattern signal and the smallest step angle of shift is limited by the small size stepping motor which is obliged to be used because of the limited space in home use sewing machines and it cannot be adjusted to follow the continuous pattern of the needle bight and work feed required by the stitch pattern generator.